Abstract Detail

Exotic grass invasion is a major concern for land management and native plant conservation. In the Southern San Joaquin Valley of California, human-disturbed environments that are heavily invaded by exotic grasses often contain some native annual species, which we called “weedy” native species. These species apparently have the functional traits and life history characteristics to compete and persist in disturbed and invaded landscapes. We measured functional traits in four weedy plant species, as well as in five non-weedy species that occur locally, but are not well represented in invaded areas. We hypothesized that a specific weedy functional type exists that allows some species to grow in Southern California landscapes that have become dominated by the invasive grass, red brome. A principal components analysis (PCA) identified three distinct ecological clusters within the species analyzed (weedy native forbs, non-weedy native forbs, and grasses), and suggested the presence of a weedy functional type. Weedy species were different from non-weedy species in several traits (large leaf area investment, large roots, large diaspores, and germination cued specifically to Meditterranean-type climate conditions). The PCA suggested complementarity as an explanation for the co-occurrence of weedy native species and red brome, but individual trait similarities suggested that competition occurs for some resources. A field trial confirmed our weedy/non-weedy plant categorization, and showed that traits related to high leaf area investment and high competitive ability for soil resources were especially important in driving success under field conditions. Further understanding of the aggressive traits shared among weedy native species may lead to cost-effective approaches to restoring invaded landscapes.